Communications cables for outside plant typically include a core comprising transmission media disposed within a polymeric tubular member called a core tube and an enclosing sheath system. The transmission media may comprise copper conductors or optical fiber. Also typically, the sheath system includes provisions for mechanical and rodent protection, such as, for example, a metallic shield, and an overlying plastic jacket. It is well known that buried and aerial cables also are damaged by lightning strikes. Accordingly, cables desirably also should include provisions which protect the transmission media portion of the cable from lightning strikes, as well as from the aforementioned mechanical and rodent damage.
A lightning strike begins as a so-called "step leader" which moves at a fraction of the speed of light in approximately fifty meter steps from clouds to earth. The step leader is not visible to the naked eye, but it results in a jagged channel to ground, the channel being full of negative charge. When the step leader reaches the ground, the negative charge is released to earth by a return strike which is the bright, noisy part of the lightning flash to ground. Subsequent strokes may occur along the same channel within tens of milliseconds of each other.
The impedance of the strike channel is considerably higher than the impedance of objects, such as telephone plant, which may become part of the strike's path. Thus, the lightning strike may be considered as a current source. The waveshapes of lightning strike currents vary, but a large number of observations has facilitated the statistical distribution of the peak current of first and subsequent return lightning strikes. Smaller lightning strikes of less than 10 kA do not arc to buried structures. Typically, 90% of first return strikes have a peak current in excess of 6.2 kA whereas only 5% have a peak current in excess of 100 kA. See p. 33 of Lightning Protection of Aircraft published in 1990 by Lightning Technologies, Inc. and authored by F. A. Fisher, J. A. Plumer, and R. A. Perala.
Damage to the transmission media caused by a lightning strike may occur in either or both of two ways. Thermal damage, that is burning, charring and melting of components of the sheath system, is caused by the heating effects of the lightning strike and a current being carried to ground by metallic members of the core or sheath system. When lightning strikes a metallic shield, for example, the metallic shield will be subjected to ohmic heating and, if it cannot carry the current, will vaporize. This will cause any adjacent elements such as a polymeric core tube to soften or melt, thereby possibly exposing the optical fibers therein to heat and damage therefrom. In buried cables, a second mode of damage is mechanical in nature, causing crushing, localized distortion of the sheath system and large scale lateral mechanical deformations of the cable. This results from an explosive impact, sometimes called a steamhammer effect, which is caused by the instantaneous vaporization of water in the earth in a lightning channel to the cable or other effects which have not yet been explained. In addition to the crushing effect, this explosive phenomena can cause a puncture of the shield, as well as cause molten metal particles from the shield or other metallic elements of the cable to be driven into the core tube, causing damage to elements of the core. In some instances, the thermal damage mechanism may dominate whereas in others, protection from mechanical damage mechanisms may be the key to preventing detrimental lightning effects on the cables.
In order to simulate the effects of lightning strikes on buried optical fiber cables, a lightning test has been developed. The test, which is performed inside a rigid wooden box which is filled with a wet sand to simulate field conditions, provides for electrical impulse testing of optical fiber cables with specified current waveforms and peak current levels. The purpose of the test is to simulate the effects of a lightning arc at a point where it attaches to a cable and to establish the relative susceptibility of optical fiber cable to damage from such arcing. This test is referred to as FOTP-181, Lightning Damage Susceptibility Test For Fiber Optic Cables With Metallic Components and, when published, will become part of a series of test procedures included within Recommended Standard ANSI/EIA/TIA-455-A. To pass a commonly specified test, a cable sample shall not sustain damage that affects the transmission of light when the peak value of the current pulse is in a predetermined range.
After removing a cable sample from the test box, the continuity of all the optical fibers is determined. This measurement is performed by directing a high intensity light at one end of the fiber, and observing light continuity at the other end, by physical examination, or by any other optical means. Typically, any fiber discontinuity constitutes failure.
The prior art abounds with patents relating to copper core cables having a sheath system which includes provisions intended to provide lightning protection. Such a sheath system may be one comprising an aluminum shield enclosed by a carbon steel shield with each having a longitudinal seam. Such a sheath system is intended to provide protection from mechanical damage, electromagnetic interference and lightning.
Lately, optical fiber cables have made inroads into the communications cable market. Although metallic conductors generally are not used for transmission in lightguide fiber cables, metallic members are commonly used in the sheath system, for example. Consequently, some form of lightning protection is needed for optical fiber cables which include metallic members. Lightning protection is perhaps even more critical for an optical fiber cable which includes metallic members due to its relatively high capacity and the fragility of the glass fibers. Also, optical fiber cable is exposed to typical mechanical hazards such as abrasion and crushing, for example, during installation. Any element or system which is included in the cable to provide lightning protection also must be able to withstand the mechanical abuse to which it may be subjected as a result of a lightning strike.
In a somewhat recently introduced lightning protection system, a sheath system which encloses a core comprising at least one conductor such as an optical fiber, for example, and a core tube includes an inner metallic shield which has a relatively high thermal capacity and a relatively low resistivity. An outer corrugated shield encloses the inner metallic shield and has a longitudinal seam. The outer shield is a laminate which comprises a corrosion-resistant metallic material having a relatively high elongation. The outer shield is bonded to a jacket which comprises a plastic material. The corrosion-resistant metallic material has sufficient thickness, elongation and tensile strength to cause the bonded composite comprising the jacket and the outer shield to provide the cable with enhanced lightning impact resistance. However, it is desired to provide still further protection with fewer manufacturing operations for the underlying polymeric core tube.
One solution which requires fewer operations is to increase the thickness of the core tube or to include multiple layered plastic tubular members in the cable structure. However, such solutions require more plastic material and/or its manufacture is difficult to control. Another solution is to use relatively high modulus materials for the core tube, but such materials usually are prohibitive in cost and exhibit hydrolytic stability problems.
Seemingly, the prior art is devoid of an economical sheath system which provides suitable protection against lightning, as well as against mechanical hazards, particularly for small size cables such as might be used in outside plant for optical fiber communications. What is desired is a cable structure which resists degradation by lightning strikes. Of course, any solution cable should be one which does not detract from other desired characteristics of the cable sheath such as small size, low cost and waterblocking capability.